Kelly, who has been at the center of Trump's feud with Fox News, asked Cruz to comment on the party front-runner's decision to skip Thursday's contest, which has cast a shadow over the final debate before Monday's caucuses.

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Cruz, Trump's main rival to win on Monday, responded with a series of comments highlighting Trump's style of insulting his opponents.

"I am a maniac," Cruz said. "And everyone on this stage is stupid, fat and ugly.

Sen. Marco Rubio (R-Fla.) followed Cruz with a comment about how the debate and the race for the White House was about more than Trump.

"It's not about Donald Trump, he's an entertaining guy. This campaign is about a president who has systematically destroyed much of the things that make this country great," he said, referring to President Obama.

Trump is holding a separate event for wounded veterans a few miles away from the debate site in Des Moines.